1. Technical Field of the Invention
The invention relates generally to communication systems; and, more particularly, it relates to communication systems (including hard disk drive (HDD) systems) having communication channels that introduce ISI (Inter-Symbol Interference) to signal transmitted therein.
2. Description of Related Art
Data communication systems have been under continual development for many years. One such type of communication system that has been of significant interest lately is a communication system that employs iterative error correction codes. Communications systems with iterative codes are often able to achieve lower bit error rates (BER) than alternative codes for a given signal to noise ratio (SNR).
A continual and primary directive in this area of development has been to try continually to lower the SNR required to achieve a given BER within a communication system. The ideal goal has been to try to reach Shannon's limit in a communication channel. Shannon's limit may be viewed as being the data rate to be used in a communication channel, having a particular SNR, that achieves error free transmission through the communication channel. In other words, the Shannon limit is the theoretical bound for channel capacity for a given modulation and code rate.
As is known, many varieties of memory storage devices (e.g. hard disk drives (HDDs)), such as magnetic disk drives are used to provide data storage for a host device, either directly, or through a network such as a storage area network (SAN) or network attached storage (NAS). Such a memory storage system (e.g., a HDD) can itself be viewed as a communication system in which information is encoded and provided via a communication channel to a storage media; the reverse direction of communication is also performed in a HDD in which data is read from the media and passed through the communication channel (e.g., sometimes referred to as a read channel in the HDD context) at which point it is decoded to makes estimates of the information that is read.
Typical host devices include stand alone computer systems such as a desktop or laptop computer, enterprise storage devices such as servers, storage arrays such as a redundant array of independent disks (RAID) arrays, storage routers, storage switches and storage directors, and other consumer devices such as video game systems and digital video recorders. These devices provide high storage capacity in a cost effective manner.
Of the many types of communication systems that have received interest in recent years, many of them undesirably introduce some degree of ISI (Inter-Symbol Interference) to signal transmitted therein. The read (and write) channel of a hard disk drive (HDD) (which can itself be viewed as being a communication channel within a communication system) is one such communication channel that oftentimes introduces ISI.
FIG. 5 illustrates a prior art embodiment of a communication system 500 employing modulation coding. Generally speaking, information u is provided to a modulation encoder 510 that is operable to modify the information (e.g., provide a certain degree of coding therein) and output a signal shown as [u′] which is then launched into an ISI (Inter-Symbol Interference) communication channel 520. In some embodiments, the ISI communication channel 520 can be viewed as being the channel through which information is written to and read from the media of a HDD (e.g., oftentimes referred to as the “read channel” in a HDD application context). This communication channel 520 introduces undesirable ISI. The signal that is then output from the ISI communication channel 520 is shown as, y, and after typically incurring some noise, the signal, ŷ=y+noise, is then received by a Viterbi detector 530 that is operable to employ the soft output Viterbi algorithm (SOVA) to determine a soft output, [û′], that is indicative of the reliability of the information within the digital signal received from the ISI communication channel 520. For example, the Viterbi detector 530 is operable to determine whether the digital signal provided to it is reliable or not. In addition, the Viterbi detector 530 can be viewed as performing the parity decoding processing in the read path in response to the parity encoding processing (that is performed by a parity encoder) in the write path. The output from this Viterbi detector 530 as provided to a modulation decoder 540 that employs the same code as the modulation encoder 510. The output from this modulation decoder 530 is a best estimate, shown as û, of the original information, u, that has been encoded by the modulation encoder 510.
Ideally, the best estimate, shown as û, is the same as the original information, u. However, sometimes there are problems (e.g., noise, defects in the ISI communication channel 520, defects in the media in a HDD context, or other deficiencies) that prohibit an accurate estimation of information contained within the signal received from the ISI communication channel 520 that the best estimate, û, is not the same as the original information, u.